From Final Fantasy Wiki

Known as the most powerful and fearsome machina ever built, Vegnagun was created by the then machina centric city of Bevelle during the Great Machina War, 1000 years before Final Fantasy X.
The weapon was massive and powerful enough to potentially destroy all of Spira. It was also able to sense emotion, and thus know whether a certain person meant to do the weapon harm. To the disappointment of its creators, however, it would not be put in use due to its inability to tell friend from foe which made it even more dangerous.

Contents

Story

During the Machina War between Zanarkand and Bevelle, Shuyin, a draftee in the army of Zanarkand, in an attempt to save his beloved, Lenne who, as a summoner, was sent to the front lines of the war, attempted to infiltrate Bevelle's headquarters and steal Vegnagun in order to decimate Bevelle's troops. Lenne discovered Shuyin's scheme, and rushed to stop him from activating a weapon that could be as deadly to its own side as it was to the enemy's. However, both were caught and met a tragic end when they were shot dead by Bevelle's guards. Shuyin's anger and despair lived on past his death, causing him to become an unsent.

Vegnagun in gun mode.

A thousand years passed and Vegnagun still resided as a forgotten secret amongst the lower caverns of Bevelle. Shuyin, having himself resided in the Den of Woe, was awaken when the Crimson Squad entered the den. Killing them all but for four, one of which he secretly possesses, he once again began making an attempt to activate Vegnagun. Upon making his way to its resting place Shuyin physically moved the weapon to the Farplane in order to activate it without interference. With Vegnagun situated in the heart of the Farplane, the machina was capable of absorbing ungodly amounts of energy in a very short time, the same energy that Shinra described that if tapped into, would power countless cities of lights. Vegnagun would be able to absorb so much power, that its cannon would be capable of omnicide, which was the undying intention of Shuyin as he attempts to fire the weapon into the very core of the Farplane.
It took the combined teamwork of the Gullwings, LeBlanc Syndicate, Nooj and Gippal to dismantle Vegnagun piece by piece before confronting Shuyin himself.

Battle

Vegnagun's multiple parts must be dismantled in order for Shuyin to confront the party himself.

Gallery

Trivia

Vegnagun is controlled by playing an organ-like instrument placed at the top of its head, each key emitting a colored glow when pressed. Its main weapon, a gargantuan cannon, is capable of firing concentrated energy to cause untold amounts of destruction.